The present invention relates to an analog integrated circuit in which a base-diffused resistor and an ion-implanted resistor are formed on one semiconductor chip.
In a conventional analog integrated circuit, a base-diffused resistor is employed as a resistor element. In this case, two particular problems arise. The first of these is that a resistor element having a high resistance occupies a large area, and the second is that the resistance of a resistor element varies widely and therefore a resistor element for determining a circuit characteristic cannot be included in the integrated circuit (hereinafter referred to as "IC"). Accordingly, in the case where a large scale analog circuit is integrated in one LSI, the size of a semiconductor chip having LSI becomes large on account of the first problem. In other words, when a low resistance is employed to make the semiconductor chip size smaller, power dissipation becomes large. Inversely, when a high resistance is employed to reduce the power dissipation, the size of semiconductor chip becomes large. In the above case, the number of external terminals is increased on the basis of the second problem. In brief, the conventional analog IC has two problems which are difficult to solve.
There has been known a Schottky TTL IC (a digital IC) which employs an ion-implanted resistor to reduce power dissipation. An analog IC has been attempted by RCA on an experimental basis, in which an ion-implanted resistor is locally formed using an additional mask to reduce the chip size.
Now, explanation will be made of a conventional technique, with reference to FIGS. 1A and 1B.
FIGS. 1A and 1B are plan and sectional views for showing the structure of a base-diffused resistor. Referring to FIGS. 1A and 1B, an N-type semiconductor layer 2 is grown on a P-type semiconductor layer 1, and a P-type semiconductor region 3 is formed in the N-type layer 2 through base diffusion techniques. Thus, a resistor element is formed between a pair of electrodes 4A and 4B. The resistance R of the resistor element is given by the following equation: ##EQU1## where .rho..sub.s indicates a sheet resistivity.
In the case of a base-diffused resistor, the sheet resistivity .rho..sub.s is in the order of 200.OMEGA., and the resistor element must have at least a width w of about 15 .mu.m when a high dimensional accuracy is required. Accordingly, when a resistance of 10 K.OMEGA. is desired, the length l of the resistor element becomes equal to 15.times.50 .mu.m, and thus an area (on the semiconductor chip) occupied by the resistor element is considerably large.
The resistances of resistors employed in designing an analog circuit range from 100.OMEGA. to 20 K.OMEGA.. Accordingly, when a large value of sheet resistivity .rho..sub.s is employed, an area occupied by a resistor having a high resistance can be reduced, but it is difficult to form a resistor having a low resistance (for example, 100.OMEGA.).
Another problem is that the sheet resistivity .rho..sub.s varies widely. Although the sheet resistivity .rho..sub.s depends upon conditions under which base diffusion is carried out, at least .+-.20 percent variations in .rho..sub.s are unavoidable at the present level of technique. Accordingly, the resistance of the resistor formed through base diffusion techniques varies in the order of .+-.20%. It is therefore impossible in most cases to form a resistor for determining a time constant of a base-diffused resistor. For this reason, both of a resistor and a capacitor employed in a time constant circuit, which is important from the standpoint of circuit characteristic, are connected to the LSI from the outside thereof. That is, a large number of external terminals for forming time constant circuits are required.